An important and required quality of copper phthalocyanine blue pigments for use in liquid inks is viscosity characteristics as a measure of reduced viscosity and dispersion stability of the base ink for the purpose of improved productivity. Inks containing a pigment with poor dispersion stability have a high initial viscosity or, even if having a low initial viscosity, thicken over time. Such a pigment is unsuitable for the production of high-concentration base inks, which are produced with the focus on productivity and diluted and conditioned into liquid inks. Some ways to improve the dispersion stability of such a pigment include controlling the primary particles, optimizing the process for surface treatment, and so forth.
PTL 1 proposes a method that includes forming a crude phthalocyanine pigment into a pre-pigment through the process of dry milling, wet milling, salt kneading, acid pasting, or acid swelling and then treating this pre-pigment in a mixture of water and an organic solvent in combination with a phthalocyanine derivative. PTL 2 proposes a method that includes dry- or wet-milling crude copper phthalocyanine in combination with a resin and a grinding auxiliary, isolating the milled mass, and then subjecting the isolated mass to a conditioning treatment. PTL 3 proposes a pigment dispersion containing a salt of phthalocyanine sulfonic acid and rosin amine and a pigment composition containing this pigment dispersion and phthalocyanine.
With a production method or a pigment composition according to PTL 1 to 3, however, it is impossible to ensure the storage stability of the ink because the copper phthalocyanine sulfonic acid binds to the pigment or calcium contained in the process water before interacting with the organic amine or a similar material added later, resulting in an insufficient effect of treating the surface of the pigment.